<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
</head>
<body>
<h1>0</h1>
<script>
    let count =0;
    //开启定时器
    setInterval(f,1000);
    function f() {
        count++;
        let h =document.querySelector("h1");
        h.innerText =count;
    }
    //匿名开启定时器
    let timer =setInterval(function () {
        console.log(count);
        if (count==5){
            clearInterval(timer)
        }
    },1000);

    //开启只执行一次的定时器
    setTimeout(function () {
        console.log("时间到");
    },3000);
</script>
</body>
</html>